An interesting site titled Houdini Tutorials
Tailored for MathematiciansAs of Houdini 13.0, particles
are fully integrated into the dynamics workflow.
Since Houdini 12.5 OpenVDB volumes have been integrated and appear in
numerous places.
The new cloud tools use them but also the FLIP fluid tools use the
openVDB volumes to surface particles in place of the particle fluid
surface SOP. Click on the links below for more information.